Current emission control regulations necessitate reduction of pollutant species in diesel engine exhaust. These pollutants include carbon monoxide, unburned hydrocarbons, particulates or particulate matter, and nitrogen oxides (NOx). Additionally, reduction of CO2 emissions is also being increasingly mandated. The decrease in the amounts of the pollutant chemical species produced during the engine operation is achieved by an optimized operation of the internal combustion engine, pre-treatments of the fuel and fuel additives, and post-treatment processing and filtration for conversion of exhaust into harmless gasses. Both oxidation and reduction processes, as well as catalytic oxidation and reduction processes are used for improving exhaust gas chemistry. The particulates are typically reduced by equipping diesel engines with particulate traps mounted in the exhaust stream, which trap or otherwise collect particulates from the exhaust to prevent their emission to the atmosphere. Catalytic oxidizers have been proposed to reduce the emission of particulates, gaseous hydrocarbons, and carbon monoxide from diesel engines. These devices do not trap the particulates, but are primarily intended to oxidize particulates while also oxidizing unburned hydrocarbons and carbon monoxide to reduce emissions of these substances.
NOx, principally NO and NO2, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. The NO2 is formed principally by the post oxidation of NO in the diesel exhaust stream. Approaches to reduce NOx include, for instance, retarding engine timing, exhaust gas recirculation, or injection of a reducing agent; however, there is typically a tradeoff between NOx and particulates. For example, exhaust gas recirculation and engine timing changes can reduce the temperature of combustion to thereby decrease NOx formation, but combustion is also affected. When NO2 is reduced due to lower temperature, particulate emissions tend to increase and conditions favoring low emissions of NOx often favor production of increased levels of CO and HC. Exhaust aftertreatment devices achieve NOx reduction by using a reductant agent, which is added to the exhaust gas entering the aftertreatment device and reacts with NOx over a catalyst in a process of selective catalytic reduction (SCR). In the selective catalytic reduction process NOx is reduced to N2 by reacting with NH3 (or urea as a source of NH3) over a selective catalyst. SCR is efficient for NOx reduction as long as the exhaust temperature is within the active temperature range of the catalyst, which is typically above 300° C.
As noted above, a trade-off exists between particulates and nitrogen oxides, that is, when combustion conditions are modified to favor low nitrogen oxides emissions, particulates are increased. For example, when NOx reduction is attempted by modifying engine timing and/or recirculating exhaust gas, particulates typically are increased. Particulate traps do not directly increase NOx, but have been associated with increased production of carbon monoxide. In addition, even with a trap, unburned hydrocarbons remain a problem. By modifying combustion to achieve more complete oxidation, decreases can be achieved for pollutants resulting from incomplete combustion, but NOx is typically increased under these conditions.
Various combustion methods, fuel treatments and additives, post-combustion exhaust treatments, traps, and exhaust filtration systems have been proposed to reduce one or more of the noted pollutants or to solve a problem related to diesel exhaust. However, the achievement of lower emissions of NOx and overall CO2 emissions reduction, while controlling particulates over reasonable periods of time, continues to present a technical challenge.